1. Field of the Invention
This invention relates to a flexographic printing press comprising a plurality of inking units and plate cylinders and at least one impression cylinder, which is driven by a coaxial central gear, which during the printing operation is in mesh with the gears of the plate cylinders, which are rotatably mounted on plate cylinder carriages, which are adapted to move toward and away from the impression cylinder on tracks, which are provided in the machine frame and extend in approximately tangential to radial directions with respect to the central gear, wherein the linking units comprise screen cylinders having screen cylinder gears in mesh with the plate cylinder gears and the screen cylinders are mounted on the inking unit carriages for movement along tracks provided on the plate cylinder carriages.
2. Description of the Prior Art
In flexographic printing presses the plate cylinders must be replaced when a printing job has been completed and the next job requires a different format. The number of plate cylinders to be replaced will depend on the number of colors to be printed. For each printing job, the plates on the several plate cylinders must be properly adjusted relative to each other so that the web traveling through the flexographic printing press will be printed upon in perfect register. This requirement gives rise to a special problem because the plate cylinders must be operatively connected to the central gear by means of the plate cylinder gears as quickly as possible for printing in perfect register so that the time required for the alteration of the press before the printing operation can be resumed, i.e., the downtime of the expensive printing press, will be minimized.
In order to eliminate the need for prolonged alterations, which can hardly performed by hand, before a new printing job is performed, a flexographic printing press which is of the kind described first hereinbefore and has been disclosed in Published German Application No. 33 05 095 comprises an adjusting mechanism, by which the plate cylinder gears can be connected outside the plate cylinders in such a manner that when the plate cylinders have been moved to the printing position by means of the plate cylinder carriages all plate cylinder gears will mesh with the central gear for printing in perfect register if the central gear has been properly adjusted before. In order to permit such preadjustment of the plate cylinders, the shaft of each plate cylinder is formed with a key slot, which extends along a generatrix of the plate cylinder so that said slot defines a line of reference for the adjustment of the plate cylinder.
The adjusting mechanism comprises a stub shaft, which is formed with a corresponding key slot, and a bushing, which has external splines and includes a centering pin, which is adapted to be slidably inserted into the key slot, and also a clamping ring. A second bushing is provided, which has internal splines and is adapted to be slidably fitted on said first bushing and is non-rotatably connected to the plate cylinder gear. A third bushing has been slidably fitted on said second bushing and secured to it by means of a clamping ring and is formed on its periphery with a notch, which determines the angular position of the third bushing relative to the centering pin. The adjusting mechanism is used first to align the centering pin with the key slot in such a manner that the splines would be in proper positions to the plates on the plate cylinders if the latter were coupled to the central gear by a slidable movement along diametral lines. But at least some of the plate cylinders are tangentially offset from the central gear and for this reason the third bushing which has been properly rotated is clamped in the adjusting mechanism to the second bushing in a position which corresponds to the phase displacement that is due to the tangential offset of the plate cylinders. When the first bushing has been clamped on the stub shaft of the associated plate cylinder, the latter can be properly adjusted relative to the central gear in the flexographic printing press in that the plate cylinder is rotated until the notch of the third bushing receives the associated detent pin, which is mounted in proper position in the machine frame.
Owing to said special adjusting mechanism and the bushings which must be aligned relative to each other and must be clamped to each other and to stub shafts of the plate cylinders that flexographic printing press is expensive and complicated and errors may occur in the adjustment of the press owing to the large number of parts to be adjusted relative to each other and to inaccuracies of the adjusting mechanism. Besides, it is not ensured that the proper plate cylinders will be mounted in the associated bearings but the positions of the plate cylinders may be interchanged so that trouble may arise.